Typically, an entire document is scanned by a digital scanner using a single scanning system, either a single full-width array carriage or a half-rate/full-rate carriage scanning mechanism. One example of a digital scanner is a platen scanning system where the document is stationary and the scanning system moves. In the platen scanning system, a single mobile carriage is utilized to scan the entire document. The carriage assembly, conventionality, includes a full-width CCD array system and a light source. The full-width CCD array receives light reflected from the document and converts the light into electrical signals representing the intensity of the received light. The electrical signals are then passed on to an electronic subsystem which performs the necessary image processing operation so as to prepare the image data either for display on a display devices such as a CRT, for storage by a storage device such as a CD-ROM, or for recording or printing on a medium such as a document.
Another example of a digital scanner is a constant velocity transport (CVT) scanning system where the document moves and the scanning system is stationary. In the CVT scanning system, a single stationary scanning system is utilized to scan the entire document wherein the document is moved past the scanning system. This scanning system, conventionality, includes a full-width CCD array system and a light source. As in the platen scanning system, the full-width CCD array receives light reflected from the document and converts the light into electrical signals representing the intensity of the received light. The electrical signals are then passed on to an electronic subsystem which performs the necessary image processing operation so as to prepare the image data either for display on a display devices such as a CRT, for storage by a storage device such as a CD-ROM, or for recording or printing on a medium such as a document.
A disadvantage with these scanning systems has been the inability of these systems to provide a high speed process which is also capable of providing a high resolution signal. Both high speed and high resolution is required in many applications to make such a system feasible, especially where large volumes of documentary material must be scanned at a high rate of speed with sufficient detail for reproduction of the documentary material with a high degree of resolution.
The conventional platen scanning system, described above, is capable of scanning a document at low or intermediate speeds with relatively high resolution, but cannot adequately scan a document at very high speeds with this high resolution. Moreover, a conventional platen scanning system is limited in speed because the physics in controlling the scanning optics become too difficult at very high speeds.
For example, a half-rate/full-rate system can scan up to around 55 copies per minute, depending on the inter document gap time and sampling resolution as illustrated in FIG. 1. FIG. 1 shows a graphical representation of the relationship between the scans per minute and the sensor pixel rate for a half-rate/full-rate scanning system. The graph shows that the top scanning rate for a half-rate/full-rate system is approximately 55 scans per minute at a resolution of 400 dots per inch by 400 lines per inch. However, if one increases the resolution to 400 dots per inch by 600 lines per inch, the top scanning rate for a commercial half-rate/full-rate scanning system is approximately 42 scans per minute. Thus, the conventional half-rate/full-rate scanning system cannot realize both high speed and high resolution.
On the other hand, a full-width array scanning system has parallel channels that allow the sensors on the full-width CCD array system to operate at a higher data rate than the typical CCD sensor associated with a half-rate/full-rate system. Such a full-width array scanning system are capable of producing high resolution scans of an image; however, the speed of the full-width array scanning system is limited by the speed it can travel or the document can be passed thereby wherein the speed is limited by the actual physics in controlling the scanning mechanism. Presently, the limit of the speed of a scanning mechanism is around 20 to 25 inches per second. Thus, the top rate for a full-width array scanning system, as illustrated in FIG. 2, is between 75 and 90 copies per minute. Examples of such systems are disclosed in U.S. Pat. No. 4,631,598 and U.S. Pat. No. 4,972,273. The entire contents of both U.S. Pat. No. 4,631,598 and U.S. Pat. No. 4,972,273 are hereby incorporated by reference.
Although a full-width CCD array system can achieve 90 scans per minute with a relatively high resolution, this speed is still slower than conventional high speed digital printers which can produce from 135 to 180 prints per minute. In other words, to develop a high speed, high resolution digital copier, the speed of the digital scanner should be approximately equal to the printing speed of the system reproducing the copies, otherwise the digital copier cannot operate with full efficiency. Therefore, it is desirable to develop a high speed high resolution, high resolution scanner which is capable of scanning documents at a speed equivalent to that which a high speed high resolution printing system with relatively high resolution.